Refactor Spies data, introducing Spy::Dataspyable .

Introduce `Spy::Dataspyable`, which makes a Spy enumerable and adds
`#[]` access of Spy data. Remove `:data` reader from all Spy classes,
which should instead use `#[]`. Change `Spy::Workers|Queues` to store
as arrays rather than hashes, mirroring `Spy::Retries|Schedules`.
Refactor Panels, updating to work with Spies [] access.

Store data for each `Display::Panel` within `@spies`, refreshing spies
within `#refresh`, and removing explicit
`Display::Panels::Workers|Queues|Retries|Schedules#refresh`. Update
`#structure`s to use the `#[]` method of data access, thereby removing
use of `rescue nil`.
Merge branch 'refactor-spy-data'